The invention relates to a method and apparatus for adapting slip threshold values for a propulsion slip and/or braking slip control system to the type or condition of the tires of a motor vehicle.
German Offenlegungsschrift No. 3,545,652 discloses a device for controlling torque output in response to slip threshold value signals. In this device, a mean slip threshold is substituted by a lower value, when the vehicle travels faster than a predetermined speed in a curve and the vehicle longitudinal acceleration is within a specific range, for providing a propulsion slip control for the vehicle. Conversely, the means slip threshold is substituted by a higher value when the vehicle likewise travels in a curve and the vehicle longitudinal acceleration is above the above-mentioned range. An adaptation of the slip threshold to the stability required for travelling curves is therefore detectable in a sense when travelling curves. The coefficient of friction, i.e., the grip of the vehicle relative to the nature of the road just being traveled, is left out of consideration, so that a changeover is made to a lower slip threshold value in the case of dry gripping road, for example, although this is not actually necessary if the actual coefficient of friction of the road was considered.
A variation of slip threshold values has been known only in the case cf propulsion slip control systems adaptable to specific driving states and/or to improve traction and/or stability. No attention had been paid to different equipment of the vehicle as regards the tires, although vehicles with both propulsion slip and braking slip control systems react very differently to one and the same road, according to whether they are equipped with summer or winter tires, or with new or worn tires.
Constant slip values which had to be coordinated with the least favorable case, smooth ice, are used predominantly in known control systems. However, these slip thresholds are too low for a higher coefficient of friction, so that only slight accelerations are possible and the propulsion slip control system is switched on with unnecessary frequency. Different sets of tires and the vehicle reactions which they modify are not taken into consideration.
It is therefore the object of the invention to disclose a method which can better predetermine slip thresholds associated with the existing tires of a motor vehicle. This creates optimum control conditions in both propulsion slip and braking slip control systems. It is also the object of the invention to disclose an apparatus for performing this method.
The object of the invention concerns a method of adapting slip threshold values for a propulsion slip and/or braking slip control system to existing tires of a motor vehicle by utilizing a curve of the attainable vehicle acceleration (a.sub.x) of a specific vehicle at vehicle standstill (v.sub.F =0), as determined from the speeds of the non-driven wheels, determined as a function of the coefficient of friction (.mu.). This curve is extended into a graph of the coefficient of friction (.mu.) as a function of vehicle speed (v.sub.F) and of vehicle acceleration (a.sub.x), taking into consideration the resistances to motion which are a function of vehicle speed (v.sub.F). A measurement window associated with a specific coefficient of friction (.mu..sub.o) is established in this graph. Then, a basic slip threshold value (.mu..sub.o) to be associated with the specific coefficient of friction (.mu..sub.o) is substituted by the slip value (.mu..sub.l) occurring at the driven wheels, when the motor vehicle attains a driving state defined by the measurement window, with the vehicle travelling straight ahead (.beta.=0), the driven wheels of the vehicle exhibiting no speed differential (.DELTA..nu.H), the acceleration (a.sub.R) of the driven wheels corresponding to the vehicle acceleration (a.sub.x), and when no propulsion slip or braking slip control occurs. The respective slip threshold value (.lambda..sub.s) is thus obtained which corresponds to the corrected slip value (.lambda..sub.l) associated with the specific coefficient of friction (.mu..sub.o).
It is possible by this method to adapt the slip thresholds automatically to the respective tires during travel and thus to create optimum control conditions.